Pyrithione (also known as 1-Hydroxy-2-pyridinethione, 2-pyridinethiol-1-oxide, 2-mercaptopyridine-N-oxide, pyridine-2-thione-N-oxide, pyridinethione-N-oxide, 2-pyridinethione, pyridinethione, or simply “PT”) has been noted for its bactericidal and fungicidal activities. Pyrithione is a bidentate ligand that forms stable complexes with most transitional metals. Metallization of pyrithione often results in highly augmented biocidial activities. Metal salts of pyrithione, such as for example, sodium pyrithione, magnesium pyrithione, barium pyrithione, bismuth pyrithione, strontium pyrithione, copper pyrithione, zinc pyrithione, cadmium pyrithione, and zirconium pyrithione, are widely used as fungicides and bactericides in a broad spectrum of commercial products, such as metalworking fluids, lubricants, paints, cosmetics and toiletries.
Zinc pyrithione (or “ZPT”) is especially useful as a broad-spectrum anti-microbial agent and preservative. It is active against both gram-positive and gram-negative bacteria, as well as fungi and yeasts. Therefore, ZPT has been used in various personal care compositions, such as for example, anti-dandruff shampoos, hair conditioners, leave-on tonics, and anti-microbial foot powders. Bar soap is a popular product form for cleansing. A bar soap comprising ZPT is particularly desirable for its broad-spectrum anti-microbial efficacy.
ZPT is a crystalline solid that can generally be mixed into a soap composition from either a powder form, or from a liquid dispersion. Powder form can be difficult to handle because they can generate dust during dosing and mixing. As a result, powder form may require complex and expensive dust handling equipment. Although liquid dispersions are generally easier to handle, they can have their own complexities for shipping, storing and processing. For example, liquid slurries must be regularly stirred during processing to avoid settling, and accurately dosed in every manufacturing line. Additionally, liquid slurries are usually maintained in a system requiring extensive cleaning, sanitizing and waste-water recovery, and may even require a separate preservative just for the liquid slurry. Therefore, a need exist for a convenient means to dose ZPT into a soap composition that can overcome these difficulties.